The invention is directed to a method of producing hydrogen (H2) by vapor- and condensed liquid-phase reforming of oxygenated hydrocarbons.
Fuel cells have emerged as one of the most promising new technologies for meeting future global energy needs. In particular, fuel cells that consume hydrogen are proving to be environmentally clean, quiet, and highly efficient devices for power generation. However, while hydrogen fuel cells have a low impact on the environment, the current methods for producing hydrogen require high-temperature steam reforming of non-renewable hydrocarbon fuels. Further still, these high-temperature methods produce significant amounts of polluting emissions and greenhouse gases such as carbon dioxide (CO2).
A key challenge for promoting and sustaining the vitality and growth of the fuel cell industry (as well as the entire industrial sector of society) is to develop efficient and environmentally benign technologies for generating fuel, such as hydrogen, from renewable resources. Notably, if hydrogen fuel for consumption in fuel cells can be generated efficiently from renewable sources, then non-renewable resources such as petroleum feedstocks can be used for other, more beneficial, and less environmentally deleterious purposes. Moreover, the generation of energy from renewable resources such as biomass, reduces the net rate of production of carbon dioxide, an important greenhouse gas that contributes to global warming. This is because the biomass itself, i.e., plant material, consumes carbon dioxide during its life cycle.
At present, the vast majority of hydrogen production is accomplished via steam reforming of a hydrocarbon (usually methane) over a suitable catalyst. Conventional steam reforming takes place at considerably elevated temperatures, generally from about 400xc2x0 C. to 700xc2x0 C. or even higher (673 to 937 K and higher).
The net desired steam reformation reaction of a hydrocarbon is shown in reaction (1). The reaction requires a catalyst, conventionally a nickel-based catalyst on a modified alumina support.
CxH2x+2+xH2Oxe2x86x92xCO+(2x+1)H2xe2x80x83xe2x80x83(1)
The nickel catalyst is sensitive to sulfur poisoning, which can be problematic. Hydrocarbon feedstocks produced from petroleum contain a significant amount of sulfur. Therefore, the hydrocarbon reactants must have the contaminating sulfur removed prior to undergoing steam reforming.
Conventional steam reforming is generally followed by one or more water-gas shift (WGS) reactions (reaction (2)) that take place in a second and perhaps a third reactor.
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(2)
The WGS reaction uses steam to convert the carbon monoxide produced in reaction (1) to carbon dioxide and hydrogen. The WGS reaction is thus used to maximize the production of hydrogen from the initial hydrocarbon reactants.
An entire, and typical, prior art process for the steam reformation of methane is illustrated schematically in FIG. 1. The hydrocarbon feedstock is first desulfurized at 10. The desulfurized feedstock is then subjected to a first high-temperature, vapor-phase reforming reaction in a first high-temperature reaction chamber at 12. As noted earlier, this reaction generally uses a nickel-based catalyst. The products of the reaction at 12 are then swept into a second reactor for a first WGS reaction 14. This first WGS reaction takes place at approximately 300xc2x0 C. to 350xc2x0 C., using an iron catalyst. The products of the reaction at 14 are swept into a third reactor for a second WGS reaction 16. This second WGS reaction takes place a reduced temperature of from about 200xc2x0 C. to 250xc2x0 C. The products of the reaction at 16 are then passed through a separator 18, where the products are separated into two streams: CO2 and H2O(the water which is pumped back into the reaction cycle at the beginning) and CO and H2. The CO and H2 stream from the separator 18 may also be subjected (at 20) to a final methanation reaction (to yield CH4 and H2) or an oxidation reaction to yield CO2 and H2.
It has been reported that it is possible to produce hydrogen via steam reformation of methanol at temperatures near 277xc2x0 C. (550 K). See B. Lindstrom and L. J. Pettersson, Int. J Hydrogen Energy 26(9), 923 (2001), and J. Rostrup-Nielsen, Phys. Chem. Chem. Phys. 3, 283 (2001). The approach described in these references uses a copper-based catalyst. These catalysts, however, are not effective to steam reform heavier hydrocarbons because the catalysts have very low activity for cleavage of Cxe2x80x94C bonds. Thus, the Cxe2x80x94C bonds of heavier hydrocarbons will not be cleaved using these types of catalysts.
Wang et al., Applied Catalysis A: General 143, 245-270 (1996), report an investigation of the steam reformation of acetic acid and hydroxyacetaldehyde to form hydrogen. These investigators found that when using a commercially available nickel catalyst (G-90C from United Catalysts Inc, Louisville, Ky.), acetic acid and hydroxyacetaldehyde can be reformed to yield hydrogen in high yield only at temperatures at or exceeding 350xc2x0 C. Importantly, the nickel catalyst was observed to deactivate severely after a short period of time on stream.
A hydrogen-producing fuel processing system is described in U.S. Pat. No. 6,221,117 B1, issued Apr. 24, 2001. The system is a steam reformer reactor to produce hydrogen disposed in-line with a fuel cell. The reactor produces hydrogen from a feedstock consisting of water and an alcohol (preferably methanol). The hydrogen so produced is then fed as fuel to a proton-exchange membrane (PEM) fuel cell. Situated between the reactor portion of the system and the fuel cell portion is a hydrogen-selective membrane that separates a portion of the hydrogen produced and routes it to the fuel cell to thereby generate electricity. The by-products, as well as a portion of the hydrogen, produced in the reforming reaction are mixed with air, and passed over a combustion catalyst and ignited to generate heat for running the steam reformer.
Conventional steam reforming has several notable disadvantages. First, the hydrocarbon starting materials contain sulfur which must be removed prior to steam reformation. Second, conventional steam reforming must be carried out in the vapor phase, and high temperatures (greater than 500xc2x0 C.) to overcome equilibrium constraints. Because steam reformation uses a considerable amount of water which must also be heated to vaporization, the ultimate energy return is far less than ideal. Third, the hydrocarbon starting materials conventionally used in steam reforming are highly flammable. The combination of high heat, high pressure, and flammable reactants make conventional steam reforming a reasonably risky endeavor.
Thus, there remains a long-felt and unmet need to develop a method for producing hydrogen that utilizes low sulfur content, renewable, and perhaps non-flammable starting materials. Moreover, to maximize energy output, there remains an acute need to develop a method for producing hydrogen that proceeds at a significantly lower temperature than conventional steam reforming of hydrocarbons derived from petroleum feedstocks. Lastly, there remains a long-felt and unmet need to simplify the reforming process by developing a method for producing hydrogen that can be performed in a single reactor.
The invention is directed to a method of producing hydrogen via the reforming of an oxygenated hydrocarbon feedstock. The method comprises reacting water and a water-soluble oxygenated hydrocarbon having at least two carbon atoms, in the presence of a metal-containing catalyst. The catalyst comprises a metal selected from the group consisting of Group VIII transitional metals, alloys thereof, and mixtures thereof.
In a first embodiment of the invention, the water and the oxygenated hydrocarbon are reacted at a temperature of from about 100xc2x0 C. to about 450xc2x0 C. More preferably, the reaction takes place at a temperature of from about 100xc2x0 C. to about 300xc2x0 C. In either instance, the reaction is run at a pressure where the water and the oxygenated hydrocarbon are gaseous.
In a second embodiment of the invention, the water and the oxygenated hydrocarbon are reacted at a temperature not greater than about 400xc2x0 C. and at a pressure where the water and the oxygenated hydrocarbon remain condensed liquids.
In the second embodiment, it is preferred that the water and the oxygenated hydrocarbon are reacted at a pH of from about 4.0 to about 10.0.
In both the first and second embodiments, it is preferred that the catalyst comprise a metal selected from the group consisting of nickel, palladium, platinum, ruthenium, rhodium, iridium, alloys thereof, and mixtures thereof. Optionally, the catalyst may also be further alloyed or mixed with a metal selected from the group consisting of Group IB metals, Group IIB metals, and Group VIIb metals, and from among these, preferably copper, zinc, and/or rhenium. It is also much preferred that the catalyst be adhered to a support, such as silica, alumina, zirconia, titania, ceria, carbon, silica-alumina, silica nitride, and boron nitride. Furthermore, the active metals may be adhered to a nanoporous support, such as zeolites, nanoporous carbon, nanotubes, and fullerenes.
The support itself may be surface-modified to remove, cap, or otherwise modify surface moieties, especially surface hydrogen and hydroxyl moieties that may cause localized pH fluctuations. The support can be surface-modified by treating it with silanes, alkali compounds, alkali earth compounds, and the like. A preferred modified support is silica that has been treated with trimethylethoxysilane.
In the second embodiment of the invention, where the water and the oxygenated hydrocarbon remain condensed liquids, the method can also further comprise reacting the water and the water-soluble oxygenated hydrocarbon in the presence of a water-soluble salt of an alkali or alkali earth metal. The addition of these salts tends to increase the overall production of hydrogen realized in the method. It is preferred that the water-soluble salt is an alkali or an alkali earth metal hydroxide, carbonate, nitrate, or chloride salt. Potassium hydroxide (KOH) is preferred.
In both the first and second embodiments, it is much preferred that the water-soluble oxygenated hydrocarbon has a carbon-to-oxygen ratio of 1:1. Particularly preferred oxygenated hydrocarbons include ethanediol, ethanedione, glycerol, glyceraldehyde, aldotetroses, aldopentoses, aldohexoses, ketotetroses, ketopentoses, ketohexoses, and alditols. From among the oxygenated hydrocarbons having six carbon atoms, glucose and sorbitol are preferred. Ethanediol, glycerol, and glyceraldehyde are the preferred oxygenated hydrocarbons from among those having less than six carbon atoms.
The invention will also function with mixed feedstocks of oxygenated hydrocarbons, that is, feedstocks containing mixtures of two or more oxygenated hydrocarbons.
The present invention thus provides methods for producing hydrogen via a low-temperature, catalytic reforming of oxygenated hydrocarbon compounds such as ethanediol, glycerol, sorbitol, glucose, and other water-soluble carbohydrates. For the purpose of the present invention, xe2x80x9creformingxe2x80x9d or xe2x80x9csteam reformingxe2x80x9d is defined as the reaction of an oxygenated hydrocarbon feedstock to yield hydrogen and carbon dioxide.
A principal advantage of the subject invention is that the oxygenated hydrocarbon reactants can be produced from renewable resources, such as biomass. Thus, the present method can be used to generate a fuel source, namely hydrogen, from an abundant and fully renewable source. Also, because living plant matter consumes carbon dioxide, the use of these feedstocks in power generation applications does not result in a net increase of carbon dioxide vented to the atmosphere.
Another equally important advantage of the present method is that it functions at a much lower temperature than conventional steam reforming of hydrocarbons. Conventional steam reforming of hydrocarbons requires operating temperatures greater than about 500xc2x0 C. (773 K). The subject method, however, is able reform aqueous solutions or gaseous mixtures of oxygenated hydrocarbons to yield hydrogen, at temperatures of from about 100xc2x0 C. to about 450xc2x0 C. in the vapor phase. More preferably still, the vapor phase reaction is run at a temperature of from about 100xc2x0 C. to about 300xc2x0 C. In the condensed liquid phase, the reaction is run at temperatures not greater than about 400xc2x0 C.
Another beneficial aspect of. the present invention is that it allows for the reforming of the oxygenated hydrocarbon and a simultaneous WGS reaction to take place in a single reactor.
Another distinct advantage of the present invention is that oxygenated hydrocarbons are far less dangerous than are the conventional hydrocarbons normally used in steam reformation. Thus, the present invention yields hydrogen from such relatively innocuous substances as ethanediol, glycerol, glucose, and sorbitol (as compared to the highly flammable methane or propane that are used in conventional reforming methods).
Still another advantage of the present invention is that when the method is carried out in the condensed liquid phase, it eliminates the need to vaporize water to steam. This is a critical concern in large-scale operations due to the high energy costs required to vaporize large amounts of water. The heat of vaporization of water is more than 2000 kJ per mole. By eliminating the need to vaporize the water, the amount of energy that must be input into the claimed method to yield hydrogen is greatly reduced. The overall energy yield, therefore, is concomitantly increased.
Thus, the subject method provides a means to convert oxygenated hydrocarbons to yield hydrogen, using a single reactor bed and reactor chamber, and at low temperatures. Such a reactor system can be fabricated at a reduced volume and can be used to produce hydrogen that is substantially free of contaminates for use in portable fuel cells or for use in applications in remote locations.
The hydrogen produced using the present invention can be utilized in any process where hydrogen is required. Thus, the hydrogen can be used, for example, as a fuel for fuel cells. The hydrogen can be used for producing ammonia, or it could be used in the refining of crude oil. The method yields a hydrogen stream that has a very low sulfur content. When low sulfur content reactants are utilized, the method yields a hydrogen stream that is substantially free of both sulfur and carbon monoxide. This type of hydrogen stream is highly suitable for use in fuel cells, where sulfur and/or carbon monoxide can poison the catalysts located at the electrodes of each fuel cell.